“Daisy, that’s all over now,” he said earnestly. “It doesn’t matter any more. Just tell him the truth — that you never loved him — and it’s all wiped out forever.”

This is the moment of truth for Gatsby. Daisy was just going off about Tom’s history of unfaithfulness, and realizes she is vulnerable right now. So he jumps in and pleads her to admit that her marriage with Tom never meant anything. If he can get her to do this, the 5 years he spent waiting for her will all be worth it.